Use <acronym> and <abbr> to expand acronyms and abbreviations using the title attribute:

Some browsers style these using dotted underlines.

Acronym v abbr? There is much debate and confusion about what exactly constitutes an acronym versus an abbreviation. Also there is different support across browsers and changing support in new standards (acronym is likely to be dropped). The best advice is to adopt one and use it consistently and to always expand the abbreviation somewhere in the main bodyof the text.